1. Field of the Invention
The present invention relates to a grain-oriented magnetic steel sheet having a low iron loss, suitable for use as an iron core material mainly for electric power transformers and rotary machines.
2. Description of the Related Art
When manufacturing a grain-oriented magnetic steel sheet, it is a common practice to use a precipitate known as an inhibitor to produce secondary recrystallized grains of a Goss orientation ({110}<001>) during final finish annealing.
Representative methods so far disclosed include a method using AlN and MnS as disclosed in Japanese Patent publication No. 40-15644 and a method using MnS and MnSe as disclosed in Japanese Patent Publication No. 51-13469, both having already been industrialized.
Apart from the above, adding CuSe and Bn as disclosed in Japanese Patent Publication No. 58-42244, using nitrides such as those of Ti, Zr and V as disclosed in Japanese Patent Publication No. 46-40855 and many other methods are known.
These methods using inhibitors are useful for stably producing secondary recrystallized grains. However, because precipitates must be finely dispersed, it is necessary that the slab heating temperature before hot rolling is at least 1,300° C. Heating of the slab to a high temperature requires higher equipment cost, and in addition, results in an increase in the quantity of scale produced during hot rolling. This leads to many problems such as a lower product yield and a more complicated equipment maintenance.
Another problem involved in the methods using inhibitors is that these inhibitors constituents, if remaining after the final finish annealing, cause deterioration of the magnetic properties. For the purpose of eliminating these inhibitor constituents Al, N, Be, Se and S, therefore, purification annealing is carried out for several hours in a hydrogen atmosphere at a temperature of at least 1,100° C. after completion of secondary recrystallization. However, purification annealing carried out at such a high temperature leads to problems of a lower mechanical strength of the steel sheet, bucking of the lower part of the coil, and a considerably lower product yield.
It is true that, as a result of this high-temperature purification annealing, the contents of Al, N, B, Se and S in steel are reduced to up to 50 ppm, respectively. These constituents are however concentrated in the forsterite film; in the interface between the film and the iron substrate these constituents remain inevitably as single substances or as compounds. Presence of these substances prevents movement of a magnetic domain wall and causes an increase in iron loss. Further, these substances present in the film/iron interface inhibit grain boundary displacement of the crystal grains directly under the film. As a result, fine grains not completely encroached by secondary recrystallized grains are often present directly below the surface layer. Presence of such fine grains also causes deterioration of the magnetic properties. Moreover, it is still difficult to eliminate Nb, Ti and V even by high-temperature purification annealing, and this is also a cause of deterioration of iron loss.
The manufacturing methods of a grain-oriented magnetic steel sheet using inhibitors faces the problem of a high cost as described above; achievement of a lower iron loss is also limited. In order to avoid these problems, we have considered use of a method not using an inhibitor.
There are known manufacturing methods of a grain-oriented magnetic steel sheet without using an inhibitor such as those disclosed in Japanese Unexamined Patent Publication No. 64-55339, No. 2-57635, No. 7-76732 and No. 7-197126. One of the features common to these techniques is that it is intended to preferentially cause growth of grains having the {110} orientation, using the surface energy as a driving force.
In order to effectively utilize the difference in surface energy, it is necessarily required to use a thin sheet for increasing the contribution of the surface. For example, the technique disclosed in Japanese Unexamined Patent Publication No. 64-55339 limits the thickness to up to 0.2 mm, and the technique disclosed in Japanese ah Unexamined Patent Publication No. 2-57635, to up to 0.15 mm. The technique disclosed in Japanese Unexamined Patent Publication No. 7-76732, not particularly limiting the thickness, reveals a very poor orientational integration as typically represented by a magnetic flux density of up to 1.700 T for B8, for a thickness of 0.30 mm according to Example 1 presented in the specification thereof. In the examples shown therein, the thickness giving a satisfactory magnetic flux density is limited to 0.10 mm. In a technique disclosed in Japanese Unexamined Patent Publication No. 7-197126 also, the thickness is not limited, but the technique is for applying a tertiary cold rolling of from 50 to 75%. The thickness necessarily becomes smaller: a thickness of 0.10 mm is proposed in an example shown in the Publication.
Most of the thicknesses of grain-oriented magnetic steel sheet now commonly use at least 0.20 mm. That is, it is difficult to obtain a product generally in use by a method using the surface energy as described above.
Further, in order to utilize surface energy, it is necessary to carry out the final finish annealing at a high temperature in a state in which the growth of surface oxides is inhibited. For example, Japanese Unexamined Patent Publication No. 64-55339 discloses a technique of using a vacuum, an inert gas, or a mixed gas of hydrogen and nitrogen as an annealing atmosphere at a temperature of at least 1,180° C. Japanese Unexamined Patent Publication No. 2-57635 recommends using an inert gas, hydrogen or a mixed gas of hydrogen and an inert gas as an annealing atmosphere at a temperature of from 950 to 1,100° C., and further, reducing the pressure of the atmospheric gases. Japanese Unexamined Patent Publication No. 7-197126 discloses a technique of carrying out final finish annealing at a temperature within a range of from 1,000 to 1,300° C. in a non-oxidizing atmosphere having an oxygen partial pressure of up to 0.5 Pa or in vacuum.
When desiring to obtain satisfactory magnetic properties by the use of surface energy, as described above, the atmosphere for the final finish annealing must be an inert gas or hydrogen, and a vacuum is suggested as a recommended condition. However, it is very difficult to use a high temperature and a vacuum simultaneously in equipment, further leading to high cost.
When utilizing surface energy, only the grains having the {110} plane are selected to grow. In other words, unlike secondary recrystallization using an inhibitor, it is not always possible that a Goss grain growth with the <001> orientation aligned with the rolling direction is selected. Magnetic properties of a grain-oriented electromagnetic steel sheet are improved only when the easy axis of magnetization <001> is aligned to the rolling direction. Satisfactory magnetic properties are unavailable in principle with the selection of grains having the {110} plane alone. That is, satisfactory magnetic properties are available only under very limited rolling conditions or annealing conditions in a method using surface energy. As a result, magnetic properties of a steel sheet available by use of surface energy are inevitably very unstable.
In a method using surface energy, furthermore, formation of a surface oxide layer must be inhibited during final finish annealing. In other words, an annealing separator such as MgO cannot be coated for annealing. It is therefore impossible to form an oxide film similar to that of an ordinary grain-oriented magnetic steel sheet manufactured using an inhibitor after final finish annealing. For example, a forsterite film is an oxide film formed on an ordinary grain-oriented magnetic steel sheet surface made by using an inhibitor upon coating an annealing separator mainly comprising MgO. The forsterite film not only imparts a tension to the steel sheet surface, but also exerts a function of ensuring adhesion of an insulating tensile coating mainly comprising a phosphate to be coated and baked. In the absence of a forsterite film, therefore, there is a large iron loss.
More specifically, the use of surface energy, known as a manufacturing technique of a grain-oriented magnetic steel sheet not using an inhibitor, encounters problems of a limited thickness of steel sheet, a poor accumulation of secondary recrystallized grain orientations, and iron loss caused by the absence of a surface oxide film.